Specific Solvation Effects on the Structures and Properties of Neutral and One-Electron Oxidized Formamidine−Formamide Complexes. A Theoretical ab Initio Study

Abstract

Ab initio quantum chemical calculations were performed for the neutral and one-electron oxidized formamidine−formamide model systems in the gas phase and in a water solution. Full geometry optimizations without any constraints on the planarity of these complexes were carried out at the HF/6-31G* level. For the neutral dimers, the solvent effects were modeled by explicit inclusion of four, six, and nine water molecules, which creates the first, intermediate, and second hydration spheres around these dimers. For one-electron oxidized systems, we have accounted for the effects of the first hydration shell water molecules. Single point calculations were also performed at the correlated MP2/6-31G*//HF/6-31G* level. The interaction and solvation energies were corrected for the basis set superposition error. It was shown that the relative stability of the neutral model formamidine−formamide complexes is quite opposite to that of the analogous adenine−uracil base pairs (J. Phys. Chem. A 1998, 102, 6167): the double proton transferred FF2 dimer becomes more stable than the zwitterionic FF3 dimer. An increase in the number of water molecules from the first to the second hydration shell results in an overestimated stability for the FF3 zwitterionic structure. For a one-electron oxidation process, the FF3(+) cation−radical dimer is the most stable dimer while the double-proton-transfer process becomes the least favorable process in both the gas phase and in a water solution.